Green tea was extracted with 80% ethanol and the extract was separated with Toyopearl HW-40 to obtain tea catechins, which consist of (-)-epicatechin (EC), (-)-epigallocatechin (EGC), (-)-epicatechin gallate (ECg) and (-)-epigallocatechin gallate (EGCg). The effect of tea catechins on methyl mercaptan (CG3SH), a main source of halitosis, was investigated in vitro. The deodorant effect of tea catechins on CH3SH was stronger than that of sodium copper chlorophyllin, which has been widely used as an oral deodorizer. Of four main components in tea catechins, the deodorant activity increased in the following order, ECECgEGCEGCg. This implies that their activities are related to their structural formula. The inhibitory effect of tea catechins on CH3SH production from saliva containing L-methionine was also evaluated. A large amount of CH3SH was produced during the incubation of saliva at 37 degrees C. for 24 h. But the incubation of saliva added tea catechins resulted in the remarkable depression of CH3SH production. Tea catechins were attempted to apply to chewing gum. By chewing gum containing tea catechins, CH3SH production was reduced significantly. These results indicated that tea catechins showed remarkable effect on the elimination of CH3SH, and chewing of gum containing tea catechins was useful to depress the bad breath